The present invention relates to a self-scanning type eddy-current contour measuring apparatus including a plurality of eddy-current distance detection sensors which are arranged in a row along a predetermined reference line so as to be close to each other and extend over a predetermined width, whereby the gap or distance of the surface of a conductive object to be measured from the reference line is measured within the predetermined width in a short period of time to measure the contour of the object to be measured.
In order that an eddy-current distance measuring apparatus of the type disclosed in U.S. Pat. No. 3,867,689, No. 3,997,835, and No. 4,030,027 may be used to measure the distance of the surface of an object to be measured and thereby to determine the contour of the object, it is necessary that the detection sensor of the eddy-current distance measuring apparatus is positioned along a predetermined fixed reference line and the distance of the surface of an object to be measured from the reference line is measured continuously or intermittently at small intervals over the entire range of the width to be measured.
In this case, by moving the sensor along the reference line through a manual drive, motor drive or the like, it is possible to continuously measure the distance of an object to be measured from the reference line. This method is effective in cases where the contour of an object to be measure does not change with time or the driving of the distance measuring sensor is not restricted in time. However, this method cannot be used in cases where the movement of the measuring sensor is restricted in time, such as when the desired speeding up is not possible, where the contour of objects to be measured changes with time, or where the object to be measured is fed in the lengthwise direction of the reference line or at right angles to it.
In such cases, by arranging a plurality of distance measuring sensors in a row so as to be adjacent to each other and to cover the extent of the desired width be measured, it is possible to measure the distance between an object to be measured and the sensors intermittently at intervals of the spacing between the sensors, and thereby to determine the contour or the like of the object. In any case of the known distance measuring methods (the oscillation method, bridge method, feedback amplification method, etc.), this type of distance measuring apparatus having a plurality of sensors comprises, as its principal parts, detecting means (a group of sensors) and arithmetic means, and the detecting means generates AC magnetic flux as a matter of course for the distance measuring apparatus of the eddy-current type. As a result, if the plurality of sensors are arranged close to each other and are actuated simultaneously, a mutual inductive action will be caused between the sensors, and consequently the sensors will interfere with each other, thus causing an error in the measured value. On the other hand, even if the sensors are each provided with the desired distance output characteristic, it is difficult to adjust the characteristics of the sensors to obtain the desired overall characteristic or the interference between the sensors in operation will make the output characteristics nonuniform, thus requiring the provision of additional circuits in the arithmetic means and thereby unavoidably making the arithmetic means complicated.
In addition, since the distance output characteristic of the distance measuring apparatus is calibrated by simultaneously actuating the plurality of sensors, even if only one of the sensors becomes faulty, this will cause the measured value of the other sound sensors to involve an error, thus making it in fact difficult to continue the measurement in this condition.
In this case, while it is conceivable to arrange the sensors so as to be apart from each other by a distance sufficient to prevent the sensors from interfering with each other, and thereby to prevent any detrimental effects of the mutual interference of the sensors, this is contrary to the intended objective of making the measuring sensor spacing as small as possible. With another method in which a magnetic shield is provided for each of the sensors, the existence of the conductive material around the sensors deteriorates the distance output characteristic, and any attempt to increase the size of the sensors to improve the distance output characteristic will considerably increase the size and weight of the apparatus which has already been increased in size and weight by the provisions of the magnetic shields. Particularly, where the available space for mounting the sensors is limited, the intended measurement will be made difficult.